In disk storage systems, the read/write head must be accurately positioned over a particular track of the magnetic disk within very precise limits. Recently, encoder-type systems have been used to accurately locate disk recording heads. Due to the accuracy required from these encoder systems, the alignment of the components thereof is extremely critical, accuracies of 200 microinches and repeatabilities of 50 microinches frequently being required. Replacement of the components in such encoder systems is periodically necessary due to burning out of the encoder light source or other malfunctions. Present encoder systems used with disk storage systems have a single illumination/detector assembly to maintain proper alignment, and the entire system must be returned to the manufacturer for replacement thereof. This is inconvenient and costly due to both the expense of repair and the loss of use of the disk storage system during the period of repair.